The present invention relates to an electrostatic spray ionization method.
Electrospray is a phenomenon that has been studied as early as 1749 when Nollet described the spray from a metallic orifice that was electrified electrostatically (Nollet J A. 1749. Recherches sur les causes particulières des phénomènes électriques. Recherches sur les causes particulières des phénomènes électriques, 1ère edn. Chez les frères Guerin, Paris). Since the 1980's, electrospray ionization (ESI) has been widely used as a powerful technique to softly ionize large compounds from solution for Mass Spectrometry (MS) analyses [Yamashita M, Fenn J B. 1984. Electrospray ion-source—another variation on the free-jet theme. Journal Of Physical Chemistry 88: 4451-59].
The principle of electrospray ionization is based first on the ejection of charged microdroplets from the tip of a capillary or microchannel and then on the formation of gas phase ions from the microdroplets. When a high potential difference is applied between an electrode placed in contact with the solution to be sprayed and a counter electrode, such as a mass spectrometer, placed in the vicinity of the tip, a fine mist of charged microdroplets is emitted from the tip of the capillary or microchannel and flies to the counter electrode. The microdroplets reduce in size during the flight by solvent evaporation and/ or by coulomb explosion to form gas phase ions representative of the species in solution.
Two mechanisms have been proposed for the formation of gas-phase ions from charged microdroplets. The first one is called Charged Residue Model (CRM). According to this model, there is a formation of extremely small microdroplets with a radius approximately equal to 1 nm and containing only one analyte ion. Solvent evaporation from such microdroplet leads to the formation of a gas-phase ion. The second mechanism considers Ion Evaporation (IE) from small and highly charged microdroplets. The model predicts that ion emission from the microdroplets becomes possible when the radius of the microdroplet is sufficiently small (r<10 nm) [Dole M, Mack L L, Hines R L, Chemistry D O, Mobley R C, et al. 1968. Molecular beams of macroions. The Journal of Chemical Physics 49: 2240-49; Mack L L, Kralik P, Rheude A, Dole M. 1970. Molecular beams of macroions. II. The Journal of Chemical Physics 52: 4977-86; Iribarne J V, Thomson B A. 1976. On the evaporation of small ions from charged droplets. The Journal of Chemical Physics 64: 2287-94].
In classical ESI-MS, a high potential is applied on an electrode in contact with the solution in a microchannel or a capillary. The mass spectrometer acts as the counter electrode. When a current flows through the electrospray emitter, electrochemical reactions occur both at the electrode/solution interface and at the ion detector. In positive ionization mode, the electrode acts as an anode where oxidation reactions take place. Conversely in negative ion mode, the electrode acts as a cathode where reduction reactions take place. These electrode reactions take place to ensure the electroneutrality of the solution [Abonnenc M, Qiao L A, Liu B H, Girault H H. 2010. Electrochemical Aspects of Electrospray and Laser Desorption/Ionization for Mass Spectrometry. In Annual Review of Analytical Chemistry, Vol 3, pp. 231-54. Palo Alto: Annual Reviews]
Recently, an inductive or induced electrospray ionization method has been reported by Cooks et al. [Huang G, Li G, Ducan J, Ouyang Z, Cooks R G. 2011. Synchronized Inductive Desorption Electrospray Ionization Mass Spectrometry. Angewandte Chemie-International Edition 50: 2503-06; Huang G, Li G, Cooks R G. 2011. Induced Nanoelectrospray Ionization for Matrix-Tolerant and High-Throughput Mass Spectrometry. Angewandte Chemie-International Edition 50: 9907-10]. A pulsed high voltage waveform is applied on an electrode 2 mm from a nanospray emitter to induce voltage inside the emitter for sample electrospray ionization. The pulsed voltage is generated by a pulsed power supply with 10-5000 Hz and 0-8 kV. In comparison with classic ESI, the high voltage is not directly applied to the sample solution during the inductive ESI, and no electrode reaction can occur. Similarly, inductive ESI by Alternating Current (AC) high voltage is reported by Zhang et al. [Peng Y, Zhang 5, Gong X, Ma X, Yang C, Zhang X. 2011. Controlling Charge States of Peptides through Inductive Electrospray Ionization Mass Spectrometry. Analytical Chemistry DOI: 10.1021/ac2024969].
Electrospray ionization is a general ionization technique that has been applied to a wide range of biomolecules and coupled to various types of mass analyzers, such as Ion Traps (IT), Time-Of-Flight (TOF), quadrupole, Fourier-Transform Ion Cyclotron Resonance (FT-ICR) and IT-orbitrap.